1. Field of the Invention
The present invention relates to a surface plasmon-generating apparatus that uses a SPASER (surface plasmon amplification by stimulated emission of radiation) diode and a method for making the surface plasmon-generating apparatus.
2. Description of the Related Art
Surface plasmons (SPs) generated in a metal surface can have a higher wave number than light propagating in air and thus are presently widely used in near-field light applications and nanophotonics, in particular, biosensing. A metal body can have a one-dimensional structure constituted by a flat surface, a two-dimensional structure constituted by stripes and the like, or a three-dimensional structure such as fine particles. In all cases, the wave number of the surface plasmons can be increased by adjusting the length of at least one axis to about several to fifty nanometers. In other words, such metal bodies offer high spatial resolution.
One of processes of generating surface plasmons in such a nano-size three-dimensional metal body (referred to as “nanometal body” hereinafter) is a process of scattering incident light coming from outside.
However, in a non-resonance state, the scattering cross-section area is significantly small and thus the coupling efficiency of incident light to the surface plasmons is low.
It has been reported that when a nanometal body undergoes geometric resonance (cavity) relative to surface plasmons, a high Q value (gain) is obtained. Examples of the structure that undergoes geometric resonance include fine spheres, rods, stripes, and grooves. When the Q value is high, coupling of evanescent light having an intense electromagnetic field generated by surface plasmons to the incident light causes condensation of light, thereby increasing the scattering cross-section area (e.g., refer to C. F. Bohren, D. R. Huffman “Absorption and Scattering of Light by Small Particles” WILEY SCIENCE PAPERBACK SERIES, pp. 340-341). In such a case also, the increase in the scattering cross-section area is about the wavelength of the light. In order to achieve a high coupling efficiency, the incident light is desirably condensed up to the diffraction limit or the positional accuracy is desirably enhanced. However, this involves a complicated system.
Usually, a laser beam is used as the incident light since the resonance frequency width is narrow. Currently, even the semiconductor laser that offers the highest electrical-to-optical power conversion efficiency (EO efficiency) displays only about 40% EO efficiency at the laser output unit in a visible light region. The EO efficiency decreases to several percent after passing through various optical systems. It is thus clear that the electric-to-surface plasmon (E-SP) conversion efficiency will also be significantly low.
It has recently been reported that when a subwavelength local electromagnetic field generated by surface plasmons is near a gain medium having an energy bandgap, energy of electron-hole pairs in the energy bandgap is transferred to the surface plasmons.
For example, when the medium is a semiconductor quantum well, “quantum well-surface plasmon coupling” occurs (refer to K. Okamoto, I. Nimi, A. Shvartser, Y. Narukawa, T. Mukaiand, A. Scherer, “Surface-plasmon-enhanced light emitters based on InGaN quantum wells. “Nature Mat. 3, 601 (2004)). The semiconductor quantum well generates electron-hole pairs by optical excitation and the energy of the electron-hole pairs is transferred to the surface plasmons. Thus, the energy hν of the surface plasmon is either equal to or lower than the energy of the excitation light. In such a state, surface plasmons are incoherent.
When a nanometal body undergoes geometric resonance, stimulated emission of surface plasmons occurs as with light in a resonator, and coherent surface plasmons can be generated. This phenomenon is called SPASER, i.e., surface plasmon amplification by stimulated emission of radiation. For example, a metal shell structure with semiconductor nanodots has been suggested (e.g., refer to M. I. Stockman, “Spasers explained” Nature Photonics 2, 327 (2008)). The gain medium generates electron-hole pairs by optical excitation and the energy of the electron-hole pairs is transferred to the surface plasmons. Thus, the energy hv of the surface plasmons is either equal to or lower than the energy of the excitation light. In any case, as with the case of resonance scattering described above, a highly accurate system for condensing incident light is desirable in order to optically excite the gain medium. Moreover, not all of incident light is absorbed by the gain medium. It is clear that even in the case where a semiconductor laser beam is used as the incident light source, the E-SP efficiency will be significantly low.
Utilization of surface plasmons can be roughly categorized into far-field systems and near-field systems. In the near-field systems in particular, surface plasmons can be used in applications that use high-intensity local light, such as various sensing, capture of fine particles and DNA, information recording devices, and near-field exposure devices. However, there are also disadvantages from the industrial viewpoint, such as that the E-SP efficiency is low, the number of system parts (optical parts in particular) is large, and highly accurate position control of optical systems is desired. Moreover, since not all of the incident light is absorbed in the gain medium, the incident light itself becomes the background noise relative to the local light, resulting in a low S/N ratio.